The present invention relates generally to railway wheels, and more particularly, to chemical steel compositions for use in manufacturing railway wheels and rims.
Numerous steel compositions are known for use in manufacturing wheels for railway cars. Recently, the volume of railway freight transportation, sometimes referred to in terms of railroad freight revenue ton-miles, is increasing to meet an increasing rail transport demand. As a result, rail cars are hauling increasingly heavier average loads. The load on the wheels of such heavy haul rail cars often approaches permissible limits, leaving little safety margin for wheel loading.
One serious problem with railway wheels is known as shattered-rim fracture. Shattered-rim fracture is a phenomenon whereby a rolling contact fatigue crack initiating at an internal defect in a wheel rim subsurface propagates from the crack ultimately to cause substantial damage to the wheel. Left unchecked, the shattered-rim fracture can cause catastrophic failure of the railway wheel and derailment of the rail car. This problem can be exacerbated as the load on the wheel or the speed of travel increase. The tendency towards shattered-rim fracture in a railway wheel also can be affected by transient effects caused by unbalanced loads, heavy braking, and other circumstances, and is becoming more of a concern with increasing railway transportation.
Shattered-rim fracture is believed to stem from internal defects in the railway wheel, such as inclusions, pores, voids, vacancies, cavities, or pinholes. The driving force of the fracture is believed to be cyclic shear stress caused by contact loading on the rim. Such stress is thought to cause Mode II (in-place shearing) crack propagation from internal defects in the wheel rim, a phenomenon in which cracks propagate from a defect in the rim when the Mode II stress intensity factor range is greater than the threshold Mode II stress intensity factor range for the rim. For this reason, railway wheels should be manufactured such that the size of internal defects is kept as small as possible, particularly in the wheel rim. While the permissible defect size for a given railway wheel may depend on factors such as diameter of the wheel and the wheel loading, in most cases, it is believed that the defect size, i.e., the diameter of the void, inclusion, or the like in the railway wheel rim, should be kept below about 1.5 mm. in size.
It is an object of the present invention to provide a railway car wheel that is resistant to shattered-rim fracture during long term heavy haul usage.
Another object is to provide a steel composition that enables the manufacture of railway wheels that are more resistant to shattered-rim fracture.
A further object is to provide a steel composition as characterized above which is less susceptible than conventional steel compositions to formation of voids, inclusions, or like defects in the finished product.
A related object is to provide a steel composition that can be consistently formed into railway wheels with voids, inclusions, and like defects in the rim that do not exceed 1.5 mm in size.
In carrying out these objects, a steel composition has been discovered, which according to the invention, consists essentially of iron, 0.40 to 0.77 wt. % carbon, 0.25 to 0.60 wt. % silicon, 0.40 to 1.20 wt. % manganese, 0.003 to 0.060 wt. % aluminum, and up to 0.0030 wt. % oxygen, with other alloying ingredients preferably not being present, or being present only in limited quantities as described in more detail hereinbelow. The invention also encompasses a railway wheel that generally comprises a rim, a hub, and a plate that connects the rim to the hub, with at least the rim, and preferably the entire wheel, being composed of the steel composition of the invention. Surprisingly, a steel alloy composed of the foregoing ingredients may be forged to form a railway wheel in which the size of internal defects, such as inclusions, voids, and the like, may be minimized and maintained within acceptable standards.